While it seems like most people are agreeing with defy, I think he's overlooking a pretty obvious solution. The only real issue with having a coach play on a team, is that they will be a DM carry. If that's the case, why can't the coach just stop trying as hard.
It's easy to take an uber in and do massive damage and gets frags against a bunch of newbies, but it's equally easy to fake a botched uber. When I'm coaching, if my team starts going up too many rounds, or my points/dominations are getting significantly ahead, I simply let myself die more/intentionally miss shots.
If you are bad at a game, you don't have a choice but to be bad. If you are good at a game, you can choose to play badly, it's not complicated. The point of a coaches presence is to be main-calling and explaining the calls that they make.
If a coach can't handle not try-harding in a newbie mix, they simply don't deserve to be coaching; it's not a place to roflstomp on kids. The same applies for returning newbies/experienced "newbies" who really shouldn't be showing up to these any more.